The influence of the surface-roughness induced scattering and the subsequent decrease of the specular reflectance is a major problem in designing X-UV optical systems such as grazing incidence mirrors used in imaging telescopes and synchrotron beam lines, or multi-layer structures recently developed as selective reflectors in the X-UV region. In this study, absolute reflectivity measurements are performed at soft-X-ray characte-ristic wavelengths and the effects of surface roughness are evidenced. The angular distri-bution of the reflected/scattered radiation (ADSR) is obtained by keeping the mirror fixed while scanning the detector. The ADSR presents a specular component and a diffuse scattering component (DSC). The DSC reveals characteristic features : side peaks and asymmetry, which can be analysed within the framework of the first-order vector theory of the roughness induced scattering.